Horizon, led by managing director Conor Ridge, has claimed that the Co Down player issued legal proceedings against the agency in September 2013 to "inflict maximum reputational damage through the media" on Mr Ridge and Colin Morrissey, his trusted lieutenant.

Horizon says Mr McIlroy knew both men were attending Mr McDowell's September 28, 2013 marriage to Kristin Stape, an interior designer from America.

Mr McIlroy, who strenuously denies Horizon's claims, issued a lawsuit against Horizon on September 27, 2013 – just the day before Mr McDowell's wedding ceremony.

Mr McIlroy (25), from Holywood, and Portrush native McDowell (35) have long enjoyed a friendly rivalry on the world's fairways and partnered each other for Ireland at golf's World Cup, Great Britain and Ireland at the Seve Trophy, and Europe at the 2010 and 2012 Ryder cups.

Both played at last week's Irish Open at Fota Island, with McIlroy making headlines by revealing in his pre-tournament interview that he'd play for Ireland and not Team GB during the 2016 Olympics in Rio.

His legal team was at pains this week to stress that their client had "no issue" with McDowell, but it is feared the forthcoming litigation could place a strain on their friendship.

This was conspicuous by his absence at Mr McDowell's sun-kissed nuptials in Baker's Bay in the Bahamas.

He says he was required for a Nike photo shoot in the UK that weekend which also featured Manchester United player Wayne Rooney.

In court papers, Horizon claims that Mr McIlroy's team referred to Mr McDowell's upcoming nuptials in emails as 'D-Day'.

This has been hotly denied by the golfer who says the 'D-Day' references related to the day when the lawsuit against Horizon would be filed.

In the legal wrangle that shows no signs of settling, McIlroy claims he was "unambiguously and consistently" led to believe by Horizon that he enjoyed the same commercial terms as Mr McDowell.

Mr McIlroy has also alleged that Mr McDowell was a shareholder in Horizon.

Horizon, which has counterclaimed against Mr McIlroy, has hit back.

It claims that documents it has obtained as part of the lawsuit show, "in graphic detail", the alleged lengths to which Mr McIlroy went in "orchestrating" the timing of his legal proceedings.

The agency says the timing of the lawsuit and various public statements at that time by Mr McIlroy "can only be interpreted as a premeditated exercise" to disadvantage Horizon and to inflict reputational damage against Mr Ridge and Mr Morrissey when he (McIlroy) knew they were going to McDowell's wedding.

Mr McIlroy has refuted Horizon's claims.

He says that he has no complaint with Mr McDowell and that the lawsuit against Horizon was filed on September 27, 2013 through a "simple gap" that emerged in his hectic touring schedule.

A spokesperson for Mr McIlroy said that there is "no eventuality" where it could see its side calling Mr McDowell as a witness in the proceedings.

The trial at Dublin's Commercial Court is due to be heard next January.

McIlroy, who is set to give evidence for at least two weeks, says that he got "markedly inferior" conditions – such as paying a higher-level commission on lucrative off-course earnings – compared to Mr McDowell.

He argued this was despite alleged representations they would get similar terms.

Mr Ridge, who has taken issue with the portrayal of Horizon as "secretive and witholding documents", has strenuously denied he made such representations.

Last Monday barrister Rossa Fanning, for Mr McIlroy, said the alleged representation by Mr Ridge was one of two "extraordinary" matters of which Mr McIlroy became aware from documents discovered for his case.

Mr McIlroy wants to amend his legal proceedings against Horizon and two other companies so as to make that claim.

Paul Sreenan SC, for Horizon and two defendant companies, said that the claim it had been represented to Mr McIlroy that his terms would be the same as Mr McDowell's was not part of his case to date.

Mr Sreenan said it was never represented to Mr McIlroy that the terms to apply to him were identical to those of Mr McDowell.

Mr McIlroy was also aware since October or November last Mr McDowell was a shareholder in Horizon, Mr Sreenan said.

Story so far

Rory McIlroy and Graeme McDowell go back a long way.

McIlroy said of McDowell in the past: "I'm great friends with Graeme. He's been like a big brother to me on tour. He really showed me the ropes out here for a couple of years, and it's great to have good friends on tour, and he's somebody I consider a very good friend.''